Grade 11 Molecular Mass Help please

Question

pottersheep · Answer

Calculate the molecular weight of a gas if 4.5L of the gas at 104KPA at 296.5K weighs 13.5g. 

The catch is I HAVE to use the forula v1p1/t2 = v2p2/t2 and NOT the ideal gas law

Please help me I don’t know how finding the VOLUME willhelp me find the molecular weiht..

.sam. · Answer

PV=nRT

pottersheep · Answer

can't:( i cant use the ideal gas law

.sam. · Answer

PV=nRT
$$PV=\frac{13.5g}{M_r}RT$$
$$M_r=\frac{13.5g}{PV}RT$$
$$M_r=\frac{13.5g}{(1.026atm)(4.5L)}(0.082\frac{atmL}{molK})(296.5K)$$
$$M_r=71.09g/mol$$

pottersheep · Answer

thank you :) that's what I did but my teacher told us to find a way to do it with the other formula but i don't even know if it's possible

.sam. · Answer

you can't use v1p1/t2 = v2p2/t2  because you're not comparing gases

pottersheep · Answer

She said convert them to STP conditions

pottersheep · Answer

If I do, I find the volume. Then I couldnt find a way to calculate the mass...

pottersheep · Answer

same here, im glad im not the only one then, maybe she's thinking wrong. Thanks for your help!

anonymous · Answer

Here's how you use the V1P1/T1 = V2P2/T2 method.

At given conditions:
1.027 atm
296.5 K
4.5 L
13.5 g

At STP conditions:
1 atm
273 K
? L
13.5 g

(4.5 L)(1.027 atm)/296.5 K = (V2)(1 atm)/273 K
V2 = 4.26 L

Since V2 is volume at STP, you may convert it to moles pretty easily with this conversion factor: 1 mol = 22.4 L

0.19 mol

13.5g/0.19 mol = 71 g/mol

pottersheep · Answer

Oh myy thanks sooo much! :D haha no wonder